In a demonstration of the Doppler effect, an observer who is standing still will most likely hear a sound's pitch increase when the source of the sound does which of the following?

• A. Moves toward the observer
• B. Moves away from the observer
• C. Spins around
• D. Changes its frequency independently

Answer: (A)

When a sound source moves toward a stationary observer, the wavefronts are compressed in front of the source. This means more sound wave crests reach the observer each second, so the observed frequency increases and the pitch sounds higher. If the source moves away, the waves spread out and fewer crests arrive per second, lowering the pitch. Spinning around or changing the emitted frequency independently aren’t about moving toward the observer, so they don’t explain the pitch increase in this scenario.